This invention relates to the field of wireless communication, and, more specifically, to the area of determining whether a transmitter transmitting on a channel of the wireless system is internal to the wireless communication system.
A wireless communications system is designed to cover a large geographic area that supports numerous mobile stations. Recently, some of these large systems are being used as a foundation for smaller systems, such as a wireless office (herein xe2x80x9cindoor systemxe2x80x9d). One such indoor system employs time-division multiple access (TDMA) technology to deliver an indoor communication environment that is flexible (especially in comparison to wireline networks) and compatible with existing digital wireless networks (herein xe2x80x9coutdoor systemxe2x80x9d).
An outdoor TDMA system achieves economy of scale by dividing the geographic coverage region into xe2x80x9ccells.xe2x80x9d In each cell, each operational frequency is divided into timeslots, whereby multiple users share the same frequency by broadcasting only during the assigned timeslot. A frequency and a timeslot are reused from cell to cell over the geographic regionxe2x80x94carefully engineered using distance and topography to keep wireless units in different cells from interfering with each other. Some overlap does occur, however, even with such careful planning. Therefore, the TDMA system assigns a xe2x80x9ccolor codexe2x80x9d to each mobile station. The transmitter of the mobile station encodes and sends this color code with other content in every timeslot. The receiver can then uniquely identify the transmitting mobile station when it decodes the content of the timeslot.
To achieve the capacity necessary to support the communication needs of an indoor system, both frequency reuse and TDMA technology are employed. An indoor network may be conceptually one large xe2x80x9ccellxe2x80x9d with only one control channel. Speech quality is improved by having a number of radio heads distributed throughout the coverage area. The radio heads use low-power transmitters, so that other radio heads throughout the indoor network can reuse channels.
In one example, indoor networks allocate channels by setting up a dynamic list of the xe2x80x9cbestxe2x80x9d frequencies and timeslots within each frequency. xe2x80x9cBestxe2x80x9d is defined as those channels that have the least interference from sources outside of the indoor network (generally the nearest outdoor system).
Further, as a mobile station moves around in the coverage area, the indoor system changes the radio head that the mobile station is communicating with by doing handoffs. The indoor system tries to use the same channel after the handoff as before, thus eliminating the requirement that the mobile station re-tune, as is commonly done during handoffs. This type of handoff is seamless to the mobile station, and is often referred to in literature as Soft Handoff. A prerequisite for soft handoff is that the radio heads are synchronized with each other.
The scanning receivers, potentially at each radio head, provide measurements to be used both for channel allocation and handoff decisions. The measurements could consist of received signal strength, and some sort of identity of the signal (if it is from an external interferer or from a mobile station within the office system) for each measured channel. For handoff purposes, the system verifies that the xe2x80x9cnewxe2x80x9d radio head""s scanning receiver is receiving the correct mobile (signal is from the mobile station involved in the handoff, and not some other signal) by checking the identity of the signal. For channel allocation purposes, the source of a signal is used to see if a channel is in use within the system or not.
Current scanning receivers, however, cannot accurately identify whether the source of a signal on a channel is internal or external to the system without decoding the content of each timeslot and examining the color code to determine whether the color code belongs to the indoor system. Completely decoding the content of a time slot requires demodulation, data detection and color code decoding. This requires fast, complex processors, program memory space and memory data space; all of which are contrary to the requirements of a small, inexpensive indoor system. On the other hand, energy at or above a threshold at synchronization (or other) symbols in the timeslot can not indicate whether the source of the signal is internal or external to the system.
According to a first aspect of this invention, a method is disclosed for use in a wireless communication system having a plurality of internal transmitters transmitting on a plurality of channels. The method determines whether a signal on a channel is being transmitted by a transmitter internal to the system. The method comprise the steps of receiving the signal being transmitted by a transmitter on a channel and determining a degree of correlation between a predefined field in the signal and a model of a predefined field in a signal transmitted by a transmitter synchronized the system. A determination is made that the transmitter of the signal is internal to the system if the degree of correlation is high.
According to a further aspect of this invention, the step of determining the degree of correlation comprises determining the degree of correlation between the predefined field and the signal over a predefined frequency range and a model of a predetermined field of the signal transmitted by a transmitter synchronized to the system. According to a further aspect of this invention, the step of determining the degree of correlation comprises determining the degree of correlation between the predefined field and the signal over a predefined time offset and a model of a predefined field as if the predefined field were transmitted by a transmitter synchronized the system.
According to a further aspect of this invention, a scanning radio in a wireless telephone system measures a signal""s strength and phase of symbols and determines whether the signal""s transmitter is internal or external to the system. The signal has a plurality of timeslots each having one or more predefined fields. The scanning receiver comprises a controller communicating with the wireless system and a variable frequency tuner. The variable frequency tuner includes a tunable receiver to receive a timeslot of a signal on one of a plurality of frequencies selected responsive to the controller. The receiver changes the frequency of the signal to an intermediate frequency and measures the signal""s strength.
This aspect of the invention also includes a quantitizer configured to sample the signal strength measurement and to quantitize each sample of the signal strength and a sampling device configured to sample the intermediate frequency signal and to derive a mathematical representation of the phase of the signal in each sample. The inventive receiver also includes a memory containing a stored list of mathematical representations of the phases and signal strengths of samples of the predefined fields.
This invention also includes a comparator that determines whether the transmitter of the signal is internal or external to the system by determining whether there is a high correlation between the mathematical representation of the phase of the signal in each sample and one of the stored list of mathematical representation of the phases. The comparator includes means for comparing the mathematical representation of the phase and the signal strength of a predetermined plurality of signal samples to the stored list of mathematical representations of phases and signal strengths of the unique synchronization words, wherein the transmitter of the signal is internal to the system if there is a high correlation.